1. Field of the Invention
The present invention relates to a method for preparing (hydroxymethyl)polysiloxanes having lateral hydroxymethyl groups and (hydroxymethyl)polysiloxanes having lateral and terminal hydroxymethyl groups.
2. Description of the Related Art
(Hydroxyalkyl)polysiloxanes, including (hydroxyalkyl)polysiloxane resins, incorporate as structural element units of the formula(siloxane-O)1+xSiRi2−x—Rii—OH,where Ri is an alkyl or an aryl residue, generally a methyl residue, Rii is a hydrocarbon residue, which may comprise or be substituted with heteroatoms, and which is attached to the silicon atom in the group SiRi2-x via a carbon atom, and x=0, 1 or 2.
“Heteroatom” is understood below to mean every atom except carbon and hydrogen, in particular nitrogen, oxygen, halogen, silicon, phosphorus and sulfur.
When x=0, corresponding to the structural element(siloxane-O)SiRi2—Rii—OH,these are polysiloxanes or polysiloxane resins with terminal hydroxyalkyl groups. If x=1 or 2, corresponding to the structural elements(siloxane-O)2SiRi—Rii—OHor(siloxane-O)3Si—Rii—OH,these are polysiloxanes or polysiloxane resins with lateral hydroxyalkyl groups.
The number of terminal hydroxyalkyl groups in a polysiloxane molecule is limited by the number of end groups in the molecule. The number of lateral hydroxyalkyl groups in a polysiloxane molecule, in contrast, is limited only by the number of monomer units of which the molecule is composed. For example, a linear polysiloxane molecule with ten-membered chains and two end groups can bear at most only two terminal hydroxyalkyl groups, but can bear between one up to a maximum of ten lateral hydroxyalkyl groups. Compared to terminal hydroxyalkyl groups, lateral hydroxyalkyl groups therefore enable a considerably more flexible and precisely adjustable functionalization of polysiloxanes.
The presence of Rii between the silicon atom and the depicted OH group has the effect that the bond attaching the OH group to the siloxane skeleton is stable to hydrolysis. If the OH group is reacted with other compounds, e.g. in polyaddition reactions with, for example, isocyanates or in polycondensation reactions with, for example, carboxylic acids, the bond attaching the resulting products to the siloxane skeleton will likewise be stable to hydrolysis.
The group Rii is in effect a structure-conferring factor, which co-determines not only the properties of the (hydroxyalkyl)polysiloxane but also the properties of the conversion products, which are prepared by using the (hydroxyalkyl)polysiloxane. It is especially both the mobility of Rii and the organic character of Rii which influence these properties (e.g. hardness or flammability). If, for example, the mobility of Rii and/or the organic character of a (hydroxyalkyl)polysiloxane or the conversion product thereof are to be kept to a minimum, the smallest possible Rii residues are ideal, and the choice of Rii as CH2 is particularly advantageous. A further advantage of this choice for Rii is that small structural units mean lower reaction volumes for the same amount of substance with Rii-attached OH groups and hence enhanced space-time yields both in the preparation of (hydroxyalkyl)polysiloxanes and of conversion products thereof. The CH2 group is, in this respect, the most efficient solution.
(Hydroxyalkyl)polysiloxanes, where Rii is equal to CH2, are known in the (hydroxymethyl)polysiloxanes below.
Methods for preparing (hydroxyalkyl)polysiloxanes are documented in the literature. Many methods, however, principally provide only hydroxyalkyl chains having at least two carbon atoms. U.S. Pat. No. 3,879,433, for example, describes the preparation of lateral hydroxyalkyl siloxanes by hydrosilylation of hydroxyolefins. However, only hydroxyalkyl groups having at least three carbon atoms are accessible in this manner. Only few methods are suitable for preparing (hydroxymethyl)polysiloxanes. In most cases, these are methods for preparing terminal (hydroxymethyl)polysiloxanes. However, the preparation of lateral (hydroxymethyl)polysiloxanes has also been described.
The preparation of siloxanes having lateral hydroxymethyl groups is possible by reaction of the corresponding chloromethylsiloxanes with magnesium metal, oxidation of the corresponding Grignard compound with oxygen and hydrolysis of the resulting magnesium alkoxide (U.S. Pat. No. 2,837,550). The method is expensive due to the magnesium metal required. The oxidation reaction is strongly exothermic and therefore difficult to manage on an industrial scale and not without hazard. Moreover, the lateral siloxane is only formed in low yield. Lateral hydroxymethyl groups in siloxanes are also accessible by the acid-catalysed alcoholysis of the corresponding (acyloxy)methylsiloxanes (e.g. DE 879839) or (acyloxy)methylsilanes (e.g. DE 1236505), in the latter case combined with co-hydrolysis and co-condensation with further silanes. Under these conditions, however, rearrangements of the siloxane skeleton and etherification of the hydroxymethyl groups also occur.
Additional methods for terminal hydroxymethyl groups in siloxanes have been described:                DE 879839: acid- or base-catalyzed equilibration of 1,3-bis(acyloxymethyl)tetramethyldisiloxane with further silanes with simultaneous alcoholytic ester cleavage,        DE 1213406: hydroxylation of bromomethylsiloxanes with metal hydroxides,        DE 1227456: acid-catalyzed equilibration of 1,3-bis(hydroxymethyl)tetramethyldisiloxane with further silanes,        DE 1233395: reductive cleavage of (acyloxy)methylsiloxanes with boronates in the presence of boron trifluoride.        
A common aspect of these methods is that, under the conditions described, rearrangements of the siloxane skeleton occur, particularly in the equilibration methods. Furthermore, in the case of acid-catalysis, an etherification of the hydroxymethyl groups can also occur. Moreover, boronates are costly and hazardous reagents.
Common to the methods described to date for preparing (hydroxymethyl)polysiloxanes is that, under the reaction conditions, slight rearrangements of the siloxane skeleton occur such that the methods do not result in defined products. Furthermore, the liberation of the ≡SiCH2OH groups from the corresponding precursor compounds (e.g. ≡SiCH2—Oacyl or ≡SiCH2-halogen) frequently does not proceed quantitatively and/or the corresponding ≡SiCH2OH groups react further under the reaction conditions (e.g. with HCl to ≡SiCH2Cl groups, with sulfuric acid to ≡SiCH2OCH2Si≡ groups or with hydroxides by cleavage of the Si—C bonds of the SiCH2OH groups to give Si—OH groups), such that the product does not have the theoretically expected number and concentration of ≡SiCH2OH groups. Moreover, reagent residues and/or catalyst residues in the product frequently lead to rearrangement, cleavage, condensation or equilibration of the siloxane skeleton, such that the product properties of (hydroxymethyl)polysiloxanes, which were prepared by the methods described to date, frequently change on storage. All this complicates or prevents the further processing of the (hydroxymethyl)polysiloxanes, prepared by the methods described to date, to defined conversion products and this applies especially to subsequent reactions of the SiCH2OH group.
DE 102009046254 describes a method for reacting terminal OH-functional polysiloxanes with cyclic or acyclic alkoxysilanes to give terminal (hydroxymethyl)polysiloxanes. In this method, rearrangements of the skeleton do not occur and defined products are formed.